Torsional hinged or galvanometer type mirrors provide an inexpensive alternate mechanism to rotating polygon shaped mirrors that generate a sweeping light beam suitable for use with visual displays and high speed printers. As will be appreciated by those skilled in the art, in order to print high quality images with a laser printer, the mirror which scans or sweeps the light beam across a light sensitive medium (such as a rotating drum) must scribe or follow the same path at the same velocity and with the same period for each scan or sweep. Failure to closely repeat the position and time for each scan or sweep is referred to as jitter. Torsional hinged or galvanometer mirrors are high Q resonant scanning oscillators with mirror tip speeds within a factor of 10 of the speed of sound. At these speeds, air resistance is the primary energy dissipation mechanism (drag) on the mirror. Air currents or air density fluctuations in the vicinity of the rotating mirror produce low frequency variations in the rotational amplitude. These amplitude variations create low frequency jitter in the scan beam that in turn produces artifacts in high quality printed images. A beam position sensor can be used to generate error signals proportional to the magnitude of the jitter and/or deviations from the desired velocity profile. These error signals in conjunction with a PI (Proportional plus Integral) or similar controller determine the power to the drive mechanism to suppress the amplitude variations and reduce the variations in the beam sweep. Unfortunately, galvanometer or torsional hinged mirrors have vibrational modes other than the oscillation or rotational mode that produces the scanned beam. Therefore, if the control or feedback loop produces changes in the frequency components of the drive mechanism that excites the mirrors non-rotational modes, then undesirable vibrational modes can be excited. The excitation of some vibrational modes can produce beam motion, which can in turn produce an even larger error signal. The feed back loop may then try to suppress this larger signal by further increasing the drive correction, which further exacerbates the error. This unstable process amplifies or pumps the undesirable mode and the controlled system may oscillate or even diverge. In practice, this problem is observed as an oscillation in the error signal and mirror jitter at high proportional loop gain settings.
Therefore, a torsional hinged scanning system would be advantageous. Such a system with rotational amplitude stabilization would be even more advantageous if it can be implemented with minimal changes to the existing structure of the operating system.